Semiconductor devices used for many applications may comprise millions of transistors or other devices such as resistors. There are many different types of transistors. A complementary metal oxide semiconductor (“CMOS”) device comprises metal-oxide semiconductor field-effect transistors (“MOSFET”). Bipolar junction transistors (“BJTs”), either NPN BJTs or PNP BJTs, may be used to make other types of transistors. A MOSFET utilizes a NPN and a PNP BJT may be used in various applications.
BJTs generally exhibit higher gain, higher frequency performance and lower noise compared to MOSFETs. BJTs may also be classified as lateral or vertical BJTs. A BJT typically includes an emitter, a collector, and a base. The collector is formed on a semiconductor substrate between a pair of shallow trench isolation (STI) regions, which electrically isolates the collector of the BJT from other devices located in the substrate. The base is located above the collector but underneath the emitter. The BJTs may be formed using CMOS technology, and may be formed simultaneously with other CMOS devices such as resistors.
Resistors may be formed by any type of resistive material and used as load devices within a variety of semiconductor devices. Polysilicon resistors are made of a thin layer of either intrinsic or low-doped polycrystalline silicon, which can result in a significant reduction in the device size and a high integration density.
In recent years, submicron technology for CMOS devices has made it possible to attain higher speed and performance. Similar progress is desired for BJTs formed using a silicon substrate. In particular, the silicon germanium (SiGe) BJTs have been considered highly promising, where SiGe has a narrower bandgap than silicon so as to form a hetero-junction. In the hetero-junction structure, the emitter can inject charge carriers with greater efficiency into the base. However, PNP BJT with a SiGe hetero-junction emitter has a very poor performance. Therefore, there is a need for improving the performance for SiGe BJTs.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.